Interpretive Summary: Making rainbow trout with four sets of chromosomes (4N) and crossing them with normal rainbow trout with two sets of chromosomes (2N) produces animals with three sets of chromosomes (3N) and these animals are sterile. Sterility is beneficial to eliminate reproduction with wild fish and to maintain good growth for longer periods of time. The techniques for producing 4N rainbow trout depend on a rather severe 'shock' treatment which must be very carefully timed to immediately precede the first cell division during embryonic development for a short, but defined period of time. Any alteration in timing of the treatment or of cell division will result in poor results in terms of mortality and successful induction of 4N animals. This study analyzed the timing of the first cell division in rainbow trout embryos from several populations and found there was significant variation among the populations. Results suggested the differences found were genetically based and it was recommended that treatment protocols should to be adjusted to compensate for these differences if the production of 4N fish is to be successful.

Technical Abstract:
Effective production of tetraploid, as well as homozygous gynogenetic and androgenetic rainbow trout relies on well defined protocols that utilize treatments of adequate intensity at the appropriate time to inhibit the initial mitotic division of a developing embryo. Although many protocols have been published that result in successful duplication of the chromosome set, the levels of induction and mortality vary greatly. An implicit assumption in most of the protocols developed is that the timing of the first cell division of embryogenesis does not exhibit much variation among individuals or populations. This study was undertaken to analyze the sources of variation in timing of first cleavage in rainbow trout (Oncorhynchus mykiss) embryos in order to adjust treatment protocols to maximize tetraploid induction. The first cleavage interval (FCI), or the time from fertilization to the mid-point of the appearance of first cleavage was determined at several temperatures for eggs from individual females from each of four different strains. Statistical analyses of the data did not reveal any significant differences among samples taken on different dates or among females within populations, but there was a significant difference shown among the populations. Further analyses revealed this difference was due to a significantly shorter FCI in one population. Data on two of the populations incubated at elevated temperatures showed a decreased FCI within the populations, but the differences between the populations remained. The results suggested that modification of the treatment protocols for induction of tetraploidy (gynogenesis and androgenesis) are needed to compensate for variation noted among populations if maximum induction and minimum mortality are to be achieved.